perience to be able accurately to diagnose the trouble and to 
prescribe the proper remedy. 
The purpose of this Leaflet is not to describe man}*- details 
but only briefly to review a few generalities and underlying prin- 
ciples concerning plant diseases, as well as to indicate some of 
the lines of work being carried on at the Brooklyn Botanic 
Garden. 
In the first place, as to the nature and distribution of the 
parasites themselves. The nematode worms mentioned above, or 
eelworms, as they are often called, are perhaps most harmful in 
the fields of the South and in greenhouse soils. These parasites 
are of microscopic size, and they bore into the roots of susceptible 
plants and cause knot-like, or gall-like growths. Sometimes they 
even infest leaves. It has proven extremely difficult to eradicate 
these nematode worms from infested soils, it being necessary in 
the case of greenhouses, to steam sterilize the soil. In the case 
of infested fields, it usually suffices to plant for a period of two 
or three years only those crops which are immune to the parasite, 
carefully killing at the same time all weeds and susceptible plants. 
(See U. S. Dept. Agri., Farmers’ Bulletin No 648, on “The con- 
trol of root-knot’’). Space forbids here the consideration of the 
numberless insect pests parasitic on plants. 
The slime molds are low organisms which resemble, in some 
respects, the protozoa among animals. Two very serious dis- 
eases among economic plants are caused by them — the club-root 
of the cabbage, and the powdery scab of the potato tuber, recently 
introduced into this country from Canada. (See Leaflets, 
Series II, No. 11). When the soil once becomes inoculated with 
these parasites, it is almost impossible to rid a field of them ex- 
cept by careful rotation of crops, accompanied by the burning of 
all infested material. 
Bacteria, sometimes popularly called germs, or microbes, are 
minute plants, sometimes so small that they cannot be seen with 
the highest powers of the compound microscope. Many plant 
diseases are caused by bacteria, among the best known of which 
are crown gall, mosaic disease of the tobacco, black rot of the 
cabbage, and pear blight. The crown gall organism is of especial 
interest because of its production in the plant attacked of lesions 
which resemble closely in some respects those of human cancer. 
Bacterial diseases variously affect plants. Some cause the cancer- 
like galls and tumors mentioned above; others cause leaf-spots, 
wilts, rots and blights. Some undoubtedly affect the plant 
through poisons, or toxins; others, such as those causing rots, 
probably affect mainly through digestive enzymes. In plant 
pathology, the knowledge of such matters as poisoning through 
